The present invention relates to a diffraction optical element making use of a high-order diffraction light, and an objective lens for an optical pick-up making use of such an diffraction optical element.
Conventionally, a diffraction optical element provided with a diffraction lens structure has been used in combination with a refractive lens in order to compensate for chromatic aberration, or change of characteristics due to temperature change. The diffraction lens structure is generally composed of a plurality of annular zones having minute steps, extending in a direction of an optical axis, therebetween. The diffraction lens structure may be formed on an optical element, which may be or may not be a refractive lens. In the field of an objective lens for an optical pick-up, the diffraction lens structure is formed on a refraction surface of an objective lens. In particular, a commercially produced objective lens for an optical pick-up is formed from resin using a metal mold.
The steps between the annular zones of the diffraction lens structure is determined in accordance with an order of the diffraction component and the wavelength of the light. Given that the order is m, and wavelength is xcex, the steps is determined such that an optical path difference (hereinafter referred to as ODP) of mxc3x97xcex is given between the inside and outside of each step. FIG. 6 illustrates a cross-section of a conventional diffraction lens structure using a first order diffraction light. The optical axis is located on the lower side of FIG. 6. As shown in FIG. 6, the width of an outer annular zone is smaller. In FIG. 6, broken lines having a smaller pitch represent lines extending through respective annular zones. A distance between two adjoining broken lines corresponds to an ODP of one wavelength, which is given by the step between adjoining zones. In other words, the diffraction lens structure shown in FIG. 6 provides first order diffraction light by providing the OPD of one wavelength at the steps between the annular zones. Broken lines having a longer pitch indicate a base curve of the refractive lens on which the diffraction lens structure is profiled.
FIG. 7A shows a cross-section of a conventional diffraction lens structure making use of high order diffraction light. FIG. 7B is an enlarged view of a circled portion A in FIG. 7A. A step closer to the optical axis provides an ODP of two wavelengths, and each of two steps at a peripheral area provides an OPD of three wavelengths. With this configuration, relatively strong high order diffraction components are obtained.
If a diffractive optical element is formed by resin molding, the diffraction lens structure may become dulled because of a dull shape of the mold and/or insufficient injection of resin material. Effects of such a dulled shape of the diffraction structure is larger for a greater step, and diffraction efficiency is lowered accordingly. If the step provides an OPD of one wavelength as shown in FIG. 6, the effects of dulled shape of the diffraction lens structure is relatively small, and the diffraction efficiency may not be lowered significantly. However, if the step is relatively large as shown in FIG. 7A, the degree of dulled condition is relatively large, as shown by solid line in FIG. 7B in comparison to a designed shape indicated by broken lines. As shown in FIG. 7B, if the dulled portion becomes relatively large, the diffractive efficiency is lowered significantly.
The present invention is advantageous in that it provides a diffractive optical element, which has less effects on the diffraction efficiency even though the diffractive optical element is to be designed to use high order diffraction components.
According to an aspect of the invention, there is provided a diffraction optical element, including a base element, and a diffraction lens structure including a plurality of annular zones concentrically arranged about an optical axis of the base element, minute steps extending in a direction of the optical axis being formed between the plurality of annular zones, the plurality of annular zones being formed on a surface of the base element. The plurality of annular zones include at least one narrow zone satisfying condition (1) and at least one wide zone satisfying condition (2):
xcex94Z(i) less than (1/2)xc2x7xcex94E(i)xe2x80x83xe2x80x83(1), 
and 
xcex94Z(i) greater than (3/2)xc2x7xcex94E(i)xe2x80x83xe2x80x83(2), 
wherein, i represents the order of the steps counted from the optical axis, xcex94E(i) represents an absolute value of an OPD provided by the i-th step, and xcex94Z(i) represents an absolute value of a difference between OPDs provided, with respect to a base curve, by an inner side end portion and outer side end portion of an annular zone between an i-th step and (i+1)-th step.
With the above configuration, when relatively large steps are to be formed in order to use a relatively high order diffraction light, by forming narrow zones, effects of the dulled shape can be suppressed, and higher diffraction efficiency can be achieved than a case where large steps are formed without sub-steps as in the conventional art.
Preferably, the diffraction optical element includes a plurality of wide zones and a plurality of narrow zones. If a plurality of wavelengths are used, it is preferable that the value of xcex94E(i) is substantially equal to the shortest one of wavelengths to be used. If the diffraction components of third or higher order are used, it is preferable that a plurality of narrow zones are arranged between a pair of wide zones.
Optionally, the base element may be formed of light transmissive material. In such a case, the diffraction lens structure may be configured to function as a transmissive diffraction lens.
Optionally or alternatively, the base element may be a lens having an aspherical surface. In such a case, the optical element may be used as an objective lens of an optical pick-up, which is capable of converging at least two beams having different wavelengths on at least two types of optical discs having different data recording densities, respectively. The lens surface may be divided into a common area through which a beam at a low NA, which is necessary and sufficient for an optical disc having a lower data recording density, passes and an exclusive high NA area through which a beam at a high NA, which is necessary only for an optical disc having a higher data recording density, passes. At least a part of the diffraction lens structure formed on the exclusive high NA area may include a plurality of annular zones having the wide zones and the narrow zones.
According to another aspect of the invention, there is provided a diffraction optical element, which includes a base element, and a diffraction lens structure including a plurality of annular zones concentrically arranged about an optical axis of the base element, minute steps extending in a direction of the optical axis being formed between the plurality of annular zones, the plurality of annular zones being formed on a surface of the base element. The diffraction lens structure may be configured to utilize m-th order diffraction component, m being an integer greater than one. In this case, the diffraction lens structure may include steps each of which primarily provides an optical path difference of m times a working wavelength, each of the steps minutely provides a plurality of sub-steps defined by a plurality of narrow width annular zones each providing an optical path difference of one wavelength.
According to a further aspect of the invention, there is provided an objective lens for an optical pick-up, the objective lens being capable of converging at least two beams having different wavelengths on at least two types of optical discs having different data recording densities, respectively. Such an objective lens may be configured to include a refractive lens having a positive power, a diffraction lens structure including a plurality of annular zones concentrically arranged about an optical axis of the refractive lens, minute steps extending in a direction of the optical axis being formed between the plurality of annular zones, the plurality of annular zones being formed on a surface of the refractive lens. The refraction surface of the lens may be divided into a common area through which a beam at a low NA, which is necessary and sufficient for an optical disc having a lower data recording density, passes and an exclusive high NA area through which a beam at a high NA, which is necessary only for an optical disc having a higher data recording density, passes. The annular zones formed in the exclusive high NA area include at least one narrow zone satisfying condition (1) and at least one wide zone satisfying condition (2):
xcex94Z(i) less than (1/2)xc2x7xcex94E(i)xe2x80x83xe2x80x83(1), 
and 
xcex94Z(i) greater than (3/2)xc2x7xcex94E(i)xe2x80x83xe2x80x83(2), 
wherein, i represents the order of the steps counted from the optical axis, xcex94E(i) represents an absolute value of an OPD provided by the i-th step, and xcex94Z(i) represents an absolute value of a difference between OPDs provided, with respect to a base curve, by an inner side end portion and outer side end portion of an annular zone between an i-th step and (i+1)-th step.
Optionally the value of xcex94E(i) is substantially equal to the shortest one of wavelengths to be used.